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    <h2 id="in-posix-sh-process-substitution-is-undefined">In POSIX sh,
process substitution is undefined.</h2>
<p>(or "In dash, ... is not supported." when using
<code>dash</code>)</p>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC3001.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/sh</span></span>
<span id="cb1-2"><a href="SC3001.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="cf">while</span> <span class="va">IFS</span><span class="op">=</span> <span class="bu">read</span> <span class="at">-r</span> <span class="va">n</span></span>
<span id="cb1-3"><a href="SC3001.html#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="cf">do</span></span>
<span id="cb1-4"><a href="SC3001.html#cb1-4" aria-hidden="true" tabindex="-1"></a>  <span class="va">sum</span><span class="op">=</span><span class="va">$((sum</span><span class="op">+</span><span class="va">n))</span></span>
<span id="cb1-5"><a href="SC3001.html#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="cf">done</span> <span class="op">&lt;</span> <span class="op">&lt;(</span><span class="ex">program</span><span class="op">)</span></span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<p>The easiest fix is to switch to a shell that does support process
substitution, by changing the shebang to <code>#!/bin/bash</code> or
<code>ksh</code>.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC3001.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/bash</span></span>
<span id="cb2-2"><a href="SC3001.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="cf">while</span> <span class="va">IFS</span><span class="op">=</span> <span class="bu">read</span> <span class="at">-r</span> <span class="va">n</span></span>
<span id="cb2-3"><a href="SC3001.html#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="cf">do</span></span>
<span id="cb2-4"><a href="SC3001.html#cb2-4" aria-hidden="true" tabindex="-1"></a>  <span class="va">sum</span><span class="op">=</span><span class="va">$((sum</span><span class="op">+</span><span class="va">n))</span></span>
<span id="cb2-5"><a href="SC3001.html#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="cf">done</span> <span class="op">&lt;</span> <span class="op">&lt;(</span><span class="ex">program</span><span class="op">)</span></span></code></pre></div>
<p>Alternatively, process substitution can often be replaced with
temporary files:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb3-1"><a href="SC3001.html#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/sh</span></span>
<span id="cb3-2"><a href="SC3001.html#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="va">tmp</span><span class="op">=</span><span class="st">&quot;</span><span class="va">$(</span><span class="fu">mktemp</span><span class="va">)</span><span class="st">&quot;</span></span>
<span id="cb3-3"><a href="SC3001.html#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="ex">program</span> <span class="op">&gt;</span> <span class="st">&quot;</span><span class="va">$tmp</span><span class="st">&quot;</span></span>
<span id="cb3-4"><a href="SC3001.html#cb3-4" aria-hidden="true" tabindex="-1"></a><span class="cf">while</span> <span class="va">IFS</span><span class="op">=</span> <span class="bu">read</span> <span class="at">-r</span> <span class="va">n</span></span>
<span id="cb3-5"><a href="SC3001.html#cb3-5" aria-hidden="true" tabindex="-1"></a><span class="cf">do</span></span>
<span id="cb3-6"><a href="SC3001.html#cb3-6" aria-hidden="true" tabindex="-1"></a>  <span class="va">sum</span><span class="op">=</span><span class="va">$((sum</span><span class="op">+</span><span class="va">n))</span></span>
<span id="cb3-7"><a href="SC3001.html#cb3-7" aria-hidden="true" tabindex="-1"></a><span class="cf">done</span> <span class="op">&lt;</span> <span class="st">&quot;</span><span class="va">$tmp</span><span class="st">&quot;</span></span>
<span id="cb3-8"><a href="SC3001.html#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="fu">rm</span> <span class="st">&quot;</span><span class="va">$tmp</span><span class="st">&quot;</span></span></code></pre></div>
<p>If streaming is important, the temporary file can be a named pipe,
and the producer or consumer can be run as a background job.</p>
<p>If the reading command accepts input from standard input, the process
substitution can be replaced with a regular pipe:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb4-1"><a href="SC3001.html#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="co">#!/bin/sh</span></span>
<span id="cb4-2"><a href="SC3001.html#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="ex">program</span> <span class="kw">|</span> <span class="cf">while</span> <span class="va">IFS</span><span class="op">=</span> <span class="bu">read</span> <span class="at">-r</span> <span class="va">n</span></span>
<span id="cb4-3"><a href="SC3001.html#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="cf">do</span></span>
<span id="cb4-4"><a href="SC3001.html#cb4-4" aria-hidden="true" tabindex="-1"></a>  <span class="va">sum</span><span class="op">=</span><span class="va">$((sum</span><span class="op">+</span><span class="va">n))</span></span>
<span id="cb4-5"><a href="SC3001.html#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="cf">done</span></span></code></pre></div>
<h3 id="rationale">Rationale:</h3>
<p>Process substitution is a ksh and bash extension. It does not work in
sh or dash scripts.</p>
<h3 id="exceptions">Exceptions:</h3>
<p>If you only intend to target shells that supports this feature, you
can change the shebang to a shell that guarantees support, or <a
href="ignore.html">ignore</a> this warning.</p>
<p>You can use <code># shellcheck disable=SC3000-SC4000</code> to ignore
all such compatibility warnings.</p>
<h3 id="related-resources">Related resources:</h3>
<ul>
<li>Help by adding links to BashFAQ, StackOverflow, man pages, POSIX,
etc!</li>
</ul>
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